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| Home > Publications database > Formation of carbonate mounds in the Porcupine Basin, offshore Ireland: Evaluating transport of substrates for microbial processes from deep sources using numerical simulation and organic geochemistrty |
| Home > Publications database > Formation of carbonate mounds in the Porcupine Basin, offshore Ireland: Evaluating transport of substrates for microbial processes from deep sources using numerical simulation and organic geochemistrty |
| Dissertation / PhD Thesis | PreJuSER-42596 |
2004
Forschungszenrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/161
Report No.: Juel-4116
Abstract: The goal of this study is to assess whether deep coral mound growth on the continental slope of the north Atlantic is related to active hydrocarbon leakage. The objects of interest are numerous buried and non-buried carbonate mounds in the Porcupine Basin, 200 km offshore Ireland. Mounds with a size of up to 3 km length and a maximum height of about 600 m have been found in two mound provinces, the Hovland-Magellan mound province on the northern slope and the Belgica mound province on the eastern slope of the basin. The shape of the mounds varies from mostly elongated to circular and mounds consist mainly of corals, carbonate crusts and fine grained clastic sediments. To evaluate the possible link between hydrocarbon leakage and mound growth 2D basin modelling in combination with geochemical analysis of sediments from gravity cores was used. Two north-south trending seismic lines with an intersecting east-west line cover the Hovland-Magellan mound province and reach the border of the Belgica mound area. Six close by exploration wells were used for calibration of the burial and thermal history using vitrinite reflectance, bottom hole temperatures and apatite fission track data. The temperature history was defined based on the geologic evolution of the Porcupine basin, assuming heat flow peaks in Jurassic/Cretaceous times (rifting) and in the Paleocene (Iceland plume). The magnitude of the heat flow peaks was determined using the stretching factor beta for each point of the basin. Calibration of the thermal history indicated that both the rifting and plume effects were required in order to reproduce all calibration data satisfactorily. For fluid flow simulation a correct geometric reconstruction of the modelled sections was required. The paleo-geometry was defined by means of estimated paleo-water depth profiles for each time step, which were then introduced to the model to prevent unrealistic structures during burial. Possible source rocks in the Porcupine Basin are found in the Jurassic and older strata containing mainly marine to lacustrine kerogen types. Cretaceous and Tertiary sequences contain marine to delta plain, coaly organic matter. As no source rock samples were available asphaltene kinetics determined on oil samples from the Connemara oil field were used for bulk kinetic characterisation of hydrocarbon generation. Additionally published compositional kinetic data sets were used for the simulation of oil and gas generation and petroleum phase behaviour. Modelling results indicate that Jurassic and older source rocks are mature to overmature throughout the basin. Cretaceous strata is immature to mature in the central part of the basin and immature on the flanks. The Tertiary sequence remains immature over the entire basin. Hydrocarbon generation started in Late Cretaceous times for the deepest sequences. Based on the compositional kinetic model used, the Upper Jurassic source rock generates undersaturated black oil while in the oil window, with gas condensate dominating at higher maturities. Phase separation was modelled to occur during migration at depth ranges between 2000 and 4000m. Upon phase separation the migration of the free gas phase dominates over that of the oil, such that gas is the main migrating fluid in the shallower intervals. Migration is mainly buoyancy driven and vertical, only Aptian and Tertiary deltaic layers direct hydrocarbon flow out of the basin. Modelling predictions over the entire basin indicate gas breaking through to the seafloor but without any obvious focussing of hydrocarbon leakage towards the Hovland-Magellan mound province. This lack of focussing could be a problem related to either lack of sufficient stratigraphic detail in the model or related to the inadequacy of using a 2D model for the simulation of an essentially 3D process. Map based 3D modelling of the tertiary layers indicates a coincidence of possible spill points and mound occurrence. Never the less, that the model predicts significant focussing of gas migration towards the Belgica mounds where a pinch out of Cretaceous and Tertiary layers beneath the mound area is observed. The inferred focussing of gas migration towards the Belgica mounds could, however, not be confirmed by geo-chemical methods, as no enhanced methane contents could be found in the gravity cores. Only traces of methane and light hydrocarbons were identified whereas no biomarkers for methanotrophic bacteria or migrated hydro-carbons could be observed. The results from this study indicate that a link between hydrocarbon leakage and carbonate mound growth is feasible in the case of the Belgica and the Hovland Magellan mound province using basin modelling. The geochemical analysis did not confirm these results as the gravity cores did not penetrate the Pre-Pliocene sediments and do therefore only show no active hydrocarbon seepage for the last million year.
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